Surface-mount packages have come to be widely used as electronic components that are provided in electronic devices, as the latter have grown smaller and thinner. Examples of such electronic component packages include, for instance, semiconductor packages in which an electronic component such as a semiconductor element is mounted on the surface of a substrate, and the electronic component is sealed with a resin. Specific examples include, for instance, semiconductor packages such as BOC (Board on Chip). Such electronic components adopt a structure such as that of a bimetal in which the sealing resin and the substrate are bonded to each other. As a result, deformation such as warping occurred in some instances on account of mismatches in the coefficients of thermal expansion (CTE) of the sealing resin and of the substrate. In particular, warping was likely to occur in cases where only one face of the substrate was resin-sealed. As a result, the electronic component is repeatedly flexed, on account of warping, if the electronic component is used in an environment where temperature changes are substantial. In some instances, this flexing resulted in degradation caused, for example, by formation of cracks near the surface of the substrate of the electronic component. This substrate degradation was apt to give rise to damage to the electronic component, for instance breakage of wiring on the substrate surface.
For instance, printed wiring boards, which are obtained by forming a circuit through partial removal of a metal layer, such as a metal foil, that is disposed on the surface of a metal-clad laminate, are used as substrates that are utilized in electronic components. Examples of such metal-clad laminates include, for instance, the metal-clad laminate disclosed in Patent Document 1.
Patent Document 1 discloses a metal foil-clad laminate that is formed using a prepreg that is provided with a fibrous base material, a first resin layer resulting from impregnating the fibrous base material with a first thermosetting resin composition, and one or more resin layers that are formed out of a thermosetting resin composition and are provided on the first resin layer, such that the elastic modulus of a resin film that is formed out of the first thermosetting resin composition is greater than the elastic modulus of a resin film that is formed out of a second thermosetting resin composition that forms a resin layer on an outermost surface of the prepreg, from among the one or more resin layers that are formed out of the thermosetting resin composition.
Patent Document 1 indicates that a printed wiring board can be obtained in which breakage of wiring during bending of the board is sufficiently prevented.
Examples of methods for producing a metal-clad laminate include, for instance, the method disclosed in Patent Document 2.
Patent Document 2 discloses a method for producing a metal foil-clad laminate that involves obtaining a prepreg through impregnation of a sheet-like base material with a thermosetting resin, followed by drying, overlaying a metal foil onto the surface of a layer of the prepreg, and press-forming the whole with heating, wherein the method comprises the steps of coating a roughened surface of the metal foil with a first resin composition that contains an acrylic rubber, an epoxy resin, a phenolic resin and an inorganic filler; coating the coated surface with a second resin composition having no tackiness in a dry state and having an elastic modulus higher than that of the first resin composition, followed by drying; overlaying the coated surface onto a layer of prepreg, and press-forming the whole with heating; and providing a low-elastic resin layer containing the first resin composition, having a thickness of 20 μm or greater, and an elastic modulus of 10 kgf/mm2 or lower, directly below the metal foil.
Patent Document 2 indicates that the obtained metal foil-clad laminate is appropriate as a material of printed wiring boards for surface mounting, and that it is possible to secure solder connection reliability in surface-mounted parts of low thermal expansion even after repeated thermal cycles. In Patent Document 2, thus, a low-elasticity resin layer is disposed under a metal foil in order to relieve stress that acts on solder connections.    Patent Document 1: Japanese Patent Application Publication No. 2009-275086    Patent Document 2: Japanese Patent Application Publication No. H8-244165